The basic concept underlying PAL and NTSC quadrature-modulated encoding television systems is that the same spectrum is shared by chroma and luminance information (FIG. 1).
For the sake of simplicity, the following presentation is limited to the NTSC color signal format, since it is well understood in the art that the principles hereof apply with like force and result in systems following the PAL signal format.
A typical NTSC encoder is represented by the FIG. 2 block diagram. The R (red), G (green) and B (blue) signal inputs from the camera are applied to 3 matrices. The outputs of these matrices are luminance (Y) and the two chroma components (I and Q). Luminance bandwidth is typically limited to 4.2 MH.sub.z, the I component bandwidth is typically limited to 1.3 MH.sub.z, and the Q component bandwidth is typically limited to 0.6 MH.sub.z. The I and Q components are then impressed as carrier-suppressed amplitude modulation components in phase-quadrature upon a 3.579545 MH.sub.z subcarrier. This subcarrier frequency is selected in such a way as to result in a 180.degree. phase shift from scanning line to adjacent scanning line, and from frame to frame, within the color television picture signal.
The quadrature modulated subcarrier is then added to the luminance carrier, and the resultant composite video signal is typically low pass filtered at 4.2 MH.sub.z. Addition of composite synchronization pulses, proper blanking, pedestal adjustments etc., results in a signal in accordance with the NTSC format.
A detailed examination of the frequency spectrum of an NTSC encoded color picture signal in the vicinity of the subcarrier shows the well-known "interleaving" principle (FIG. 3). Spectral rays of a typical television scene are grouped around multiples of fh (where fh= horizontal scanning frequency) for the luminance information, while chroma components are grouped around (2n+1)/2 fh (where n is an integral number). This grouping is a particularly accurate representation of the spectral appearance of vertical components of the picture, and enables separation of chroma and luminance at the receiver to be accomplished by use of a comb filter with a fair degree of discrimination between these two components.
Unfortunately, the frequency interleaving is perfect only in the case of vertical transitions in the picture. Diagonal and horizontal transitions in the picture image manifest and undesirable overlap of chroma and luminace spectra; and, the separation of the luminance and chroma components in the receiver becomes difficult and in some cases, impossible at such transitions.
As a result of imperfect separation of luminance and chroma components, certain luminance components will be misunderstood by the receiver's decoder and decoded as color. This mistake results in the well known "cross-color" pattern which is typically perceived as a moving rainbow accompanying diagonal luminance transitions, or color activity associated with luminance details.
Certain chroma components will also be misunderstood by the decoder and decoded as luminance. One or 2 lines of dots at the 3.58 MHz color subcarrier frequency will be perceptibly present in the luminance path for horizontal chroma transitions with comb filter decoders. A decoder using a 3.58 MHz trap in the luminance path exhibits either a poor frequency response, or a vertical dot pattern with vertical chroma transitions, or both.
Thus, it has become popular to process luminance and chrominance components by comb filter structures prior to their addition in an NTSC (or PAL) quadrature modulation encoder. As shown in FIG. 4, only the frequencies which are in the vicinity of the chroma subcarrier frequency are "Y" combed in the luminance path, while the entire chrominance spectrum is in effect "C" combed in the chroma path. The structure of FIG. 4 has been found to be effective in reducing significantly cross-color and cross-luminance artifacts customarily associated with the NTSC (or PAL) process. Spectral results achieved by such comb filter processes are graphed in FIGS. 5A-D.
In the situation where a comb filter processor is processing a luminance component in a color television signal path, what is happening is that the high frequencies are averaged over e.g. three lines. Luminance frequencies which happen to be in the chroma subcarrier area are averaged over three lines. When the information in the adjacent lines is arranged vertically, the average is the same as corresponding information in each line. When a transition is horizontal, there is nothing to average, and there is no problem. However, if a transition is angled at e.g. forty five degrees (45.degree.), or at some certain angle other than zero or ninety degrees, the luminance transition does not line up vertically, and the averaging of the comb filter creates a blur or smearing along its length.
The situation is not as bad as it may seem, since the NTSC format provides about forty percent (40%) shorter visual transitions at forty five degrees (45.degree.) than along either the horizontal or vertical dimension. In other words, it takes the square root of two less time for a black to white transition along the forty five degree dimension of the picture than for the same transition along a ninety degree (vertical) dimension thereof. Thus, it is possible to degrade the diagonal picture resolution somewhat by the insertion of the luminance comb filter structure without resulting degradation becoming visibly objectionable in a standard bandwidth picture and without resort to any adaptivity to control the luminance comb filter structure.
It has become popular to follow an NTSC format having a bandwidth considerably wider than the format's nominal 4.2 MHz, particularly in applications such as computer generated graphics. In such high resolution environments, including the broadcast and movie studio, and multiple generation video recording, any visible loss of resolution is noticeable and is to be avoided, if at all possible.
While it is known that the luminance comb filter structure will eliminate cross color artifacts, with resultant degradation of diagonal or a motion resolution, a hitherto unsolved need has arisen for a control mechanism which effectively removes the luminance comb filter structure from the signal processing path in those picture instances in which cross color is not perceptible and does not need to be eliminated by comb filter spectral processing of the luminance component. This control mechanism will then avoid resolution losses when picture conditions make comb filtering unnecessary.
While comb filter structures having threshold control in signal decoders have been proposed in several prior art references, none has achieved a practical solution to the problem as is achieved by the present invention, more particularly in the case of a luminance comb filter used in the luminance path of an encoder prior to its addition to chroma. Such prior examples include the present applicant's own U.S. Pat. No. 4,240,105 which provides for threshold control of a logic circuit controlling gating of comb filter processed luminance in a color decoder apparatus.